1. Field of the Invention
The present invention relates to a container made of a laminated plastic sheet and a method for manufacturing the same. More particularly, the present invention pertains to a container having a gas-barrier characteristics and a method for manufacturing the same.
2. Description of the Prior Art
It is desirable that containers for foodstuffs are capable of protecting the contents therein so that the contents do get adverse effects even after the contents have been handled for transportation and stored for an extended period of time under a room temperature, and that they are capable of being subjected to heating and cooking with a microwave oven. Further, those containers which are subjected to a retort sterilization have to be of a property which can withstand a high temperature and a high humidity to which the containers are subjected. Plastic material is preferred for manufacturing such containers because of the lower cost and readiness for the manufacture.
Conventionally, containers for foodstuffs have been manufactured by means of for example forming a sheet of a plastic material under an elevated temperature, a blow forming, or an injection molding. Since containers for foodstuffs must be harmless to the contents therein and are required to be capable of providing a barrier to liquid and gas which may penetrate from the inside of the container to the outside and to gas which may penetrate from the outside to the inside of the container, it is recommendable to manufacture the container from a laminated sheet made of a plurality of layers respectively having properties as required. A preferable method for the manufacture of the container is to form the sheet under an elevated temperature.
Conventionally, such containers have been manufactured from a sheet of a plastic material such as polystylene resin, ABS resin or polypropylene resin by forming the sheet under an elevated temperature. However, the conventional method cannot provide a container having a satisfactory gas barrier property so that the container manufactured through the conventional method has only a limited use. In order to provide a gas barrier property, it has been known to use a laminated sheet including a metal foil covered at the opposite sides by plastic layers such as layers of polypropylene layers and form the sheet into a desired configuration under an elevated temperature. The conventional method using such a laminated sheet is however limited to the manufacture of a container of a relatively shallow configuration such as a tray because of the metal foil contained in the laminated sheet. Therefore, it has not been possible to manufacture with this method a container of a deep configuration such as a cup, bowl and the like. It should further be noted that the container thus manufactured from the laminated sheet is not suitable for a cooking by a microwave oven.
It has been known that some kinds of polymer material such as polyvinylidene chloride copolymer, ethylenevinyl-alcohol copolymer has an improved gas barrier property when it is stretched. When such resin material is used for the manufacture of a container, the forming must be conducted with the resin material in a softened condition so that the stretched condition can be maintained. However, when a container is thus formed with the resin material in a softened condition, it cannot maintain its shape under a high temperature to which the container will subjected when a hot content is to be charged or when the content is being subjected to a retort sterilization. As the result, the container may be distorted under such high temperature. If the forming is carried out with the resin material in a molten condition, it will no longer be possible to maintain the stretched condition of the material so that the property of the material cannot be utilized to provide an improved gas barrier property.